1. Field of the Invention
This invention relates to rear screen projectors and particularly those which provide variable magnification.
2. Description of the Prior Art
Rear screen projectors are useful for lecture or demonstration purposes because the lecturer and the audience remain in continual facing relationship and the lecturer may illustrate his discussion with sketches, graphs or writings without being forced to turn his back on the audience as would be necessary with the customary blackboard or overhead projected image.
U.S. Pat. Nos. 2,181,133 and 2,238,267 illustrate lecture desks which include rear projection (back lit) screens facing the audience and permit the lecturer to project images from object stages located atop the desks to the screens for viewing by the audience. Such a system is useful for illustration to large audiences located some distance from the lecturer but cannot conveniently be used with small groups. In addition, the size of the unit makes transportation difficult.
Briefly stated, the present invention provides a lectern which sits atop a conventional conference table and provides a podium type structure from which a standing lecturer may project informational images to an audience seated around the table. The lectern is approximately 1.5 meters wide by 1 meter high and deep and includes a projection screen and optics which produces a 1.60X magnified image, facing the audience, of a transparent object of standard A4 paper size placed upon the upper surface of the lectern.
While the size of the projected image is suitable for most information such as drawings and graphs, the size of the projected image produced by 1.60X magnification is too small to allow typewritten material to be easily read by people in the back of the audience. It is desirable, therefore, to provide additional magnification for typewritten documents so that these documents may be easily read by all portions of the audience.
To provide this additional magnification, the optical system of the lectern should be provided with at least two magnifications, the first of which projects a full sized image of an A4 sheet on a reasonably sized screen and the second of which provides an increased magnification image of a reduced area of the stage, so that fine details of the object can be understood.
The problem with providing dual-magnification is that the lecturer cannot conveniently refocus the optical system of the lectern in switching from one magnification to another because the projection screen is located on the side of the lectern opposite the lecturer and cannot easily be seen.
U.S. Pat. Nos. 3,369,450 and 4,146,315 provide dual magnification by the imposition of an auxiliary lens adjacent the main projection lens in the optical path but both require manual axial movement of the lens system along the optical path to focus the systems at the changed magnifications and, therefore, are not suitable for the lectern described above.
U.S. Pat. Nos. 3,588,226 and 4,067,648 provide dual magnification without requiring refocusing by interchanging two complete projection lens systems when a change in magnification is desired. While this solution might be workable in the proposed lectern, the provision of two complete projection lens systems is less expensive and the transport mechanism necessary to change the entire projection lens systems is necessarily complex.
U.S. Pat. No. 3,675,998 provides a change in magnification without refocusing of the optics by axially repositioning and reversing one of the lens elements of a two lens system. The complexity of the lens transport mechanism is undesirable and the space required for the axial repositioning and reversal of the lens element would probably preclude its use in a lectern of the size described.